Systems and methods for high dynamic range imaging using crossed polarizers

ABSTRACT

According to one embodiment of the present disclosure, an imaging system includes: an image sensor including a plurality of subpixels grouped into a plurality of pixels; a polarization system including: a rotatable linear polarizer; and a polarizer mask including a plurality of polarizer filters, the polarizer filters being aligned with corresponding ones of the subpixels, the subpixels of a pixel of the plurality of pixels being located behind polarizer filters at different angles of linear polarization; and imaging optics configured to focus light from a scene onto the image sensor.

FIELD

Aspects of embodiments of the present disclosure relate to the field of imaging, and more particularly, high dynamic range imaging using crossed polarizers.

BACKGROUND

Image sensors, such as complementary metal oxide semiconductor (CMOS) digital image sensors and charge coupled device (CCD) digital image sensors generally have limited dynamic ranges (or ratios between the largest and smallest values that they can detect). When the dynamic range of a scene being imaged is larger than the dynamic range of the image sensor, parts of the scene may be overexposed (e.g., brighter than the largest value the sensor can output) or underexposed (e.g., darker than the lowest value the sensor can output). In some circumstances some parts of an image may be overexposed whereas other parts of the same image are underexposed.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the present disclosure, and therefore, it may contain information that does not form prior art.

SUMMARY

Aspects of embodiments of the present disclosure relate to systems and methods for high dynamic range imaging using crossed polarizers.

According to one embodiment of the present disclosure, an imaging system includes: an image sensor including a plurality of subpixels grouped into a plurality of pixels; a polarization system including: a rotatable linear polarizer; and a polarizer mask including a plurality of polarizer filters, the polarizer filters being aligned with corresponding ones of the subpixels, the subpixels of a pixel of the plurality of pixels being located behind polarizer filters at different angles of linear polarization; and imaging optics configured to focus light from a scene onto the image sensor.

The imaging may further include a processing circuit configured to: control the image sensor to capture an image of the scene; determine that the image includes underexposed pixels and overexposed pixels; and adjust a dynamic range of the imaging system by controlling an angle of the rotatable linear polarizer.

The processing circuit may be further configured to iteratively adjust the dynamic range of the imaging system until the dynamic range of the imaging system is equal to or larger than a dynamic range of the scene.

The processing circuit may be further configured to control a plurality of exposure settings in accordance with the dynamic range of the imaging system and the dynamic range of the scene.

The processing circuit may be further configured to compute a high dynamic range image from a plurality of polarization channels of captured image.

The rotatable linear polarizer may include an electronically controlled mechanically rotatable linear polarizer.

The rotatable linear polarizer may include a liquid crystal polarization rotator.

The rotatable linear polarizer may be in front of the imaging optics.

The rotatable linear polarizer may be behind the imaging optics.

The imaging system may further include a color filter.

The polarizer mask may include a repeating mosaic of the polarizer filters at the different angles of linear polarization, the different angles of linear polarization including a first angle 0°, a second angle of 45°, a third angle of 90°, and a fourth angle 135°, and a pixel of the pixels may include: a first subpixel behind a first polarizer filter of the polarizer mask at the first angle of 0°; a second subpixel behind a second polarizer filter of the polarizer mask at the second angle of 45°; a third subpixel behind a third polarizer filter of the polarizer mask at the third angle of 90°; and a fourth subpixel behind a fourth polarizer filter of the polarizer mask at the fourth angle of 135°.

The imaging system may further include a processing circuit configured to control the rotatable linear polarizer to rotate in a range from 0° to 22.5°.

According to one embodiment of the present disclosure, a method for capturing images includes: controlling, by a processing circuit, an image sensor of an imaging system to capture an image, the image sensor including a plurality of subpixels grouped into a plurality of pixels, the imaging system further including: a polarization system including: a rotatable linear polarizer; and a polarizer mask including a plurality of polarizer filters, the polarizer filters being aligned with corresponding ones of the subpixels, the subpixels of a pixel of the plurality of pixels being located behind polarizer filters at different angles of linear polarization; and imaging optics configured to focus light from a scene onto the image sensor; determining, by the processing circuit, that the image includes underexposed pixels and overexposed pixels; and adjusting, by the processing circuit, a dynamic range of the imaging system by controlling an angle of the rotatable linear polarizer.

The method may further include iteratively adjusting the dynamic range of the imaging system until the dynamic range of the imaging system is equal to or larger than a dynamic range of the scene.

The method may further include controlling a plurality of exposure settings in accordance with the dynamic range of the imaging system and the dynamic range of the scene.

The method may further include computing a high dynamic range image from a plurality of polarization channels of captured image.

The rotatable linear polarizer may include an electronically controlled mechanically rotatable linear polarizer.

The rotatable linear polarizer may include a liquid crystal polarization rotator.

The rotatable linear polarizer may be in front of the imaging optics.

The rotatable linear polarizer may be behind the imaging optics.

The imaging system may further include a color filter.

The polarizer mask may include a repeating mosaic of the polarizer filters at the different angles of linear polarization, the different angles of linear polarization including a first angle 0°, a second angle of 45°, a third angle of 90°, and a fourth angle 135°, and a pixel of the pixels may include: a first subpixel behind a first polarizer filter of the polarizer mask at the first angle of 0°; a second subpixel behind a second polarizer filter of the polarizer mask at the second angle of 45°; a third subpixel behind a third polarizer filter of the polarizer mask at the third angle of 90°; and a fourth subpixel behind a fourth polarizer filter of the polarizer mask at the fourth angle of 135°.

The method may further include controlling the rotatable linear polarizer to rotate in a range from 0° to 22.5°.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.

FIG. 1A is a schematic diagram depicting an imaging system including crossed polarizers according to some embodiments of the present disclosure.

FIG. 1B is a schematic diagram depicting an imaging system including crossed polarizers according to some embodiments of the present disclosure.

FIG. 1C is a schematic diagram depicting a rotatable linear polarizer implemented as an electronically controlled mechanically rotatable linear polarizer according to some embodiments of the present disclosure.

FIG. 1D is a schematic diagram depicting a rotatable linear polarizer implemented as a liquid crystal polarization rotator according to some embodiments of the present disclosure.

FIG. 2 is a graph depicting the dynamic range of an imaging system according to one embodiment of the present disclosure as a function of the angle of the rotatable polarizer.

FIG. 3 is a flowchart depicting a method for controlling an imaging system to increase the dynamic range of the imaging system based on the range of radiance of a scene being imaged by the imaging system according to one embodiment of the present disclosure.

FIG. 4A is a graph comparing the radiance range of a scene to an initial dynamic range of the imaging system before beginning adjustment.

FIG. 4B is a graph comparing the radiance range of a scene to a dynamic range of the imaging system after adjusting the rotatable polarizer according to one embodiment of the present disclosure.

FIG. 4C is a graph comparing the radiance range of a scene to a dynamic range of the imaging system after adjusting the rotatable polarizer and the exposure settings of the imaging system according to one embodiment of the present disclosure.

FIG. 5 is a schematic diagram depicting an imaging system including crossed polarizers and a color filter according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Like reference numerals designate like elements throughout the specification.

The dynamic range of an imaging device may be defined as a ratio between the brightest and darkest image irradiance that the device is capable of measuring. In general terms, the dynamic range of human vision is on the order of 4,000 times greater than the dynamic range of a conventional imaging device. Due to the limited dynamic range, images taken by the imaging device of high-contrast scenes may contain regions with overexposed or underexposed pixels, and fail to convey the true image irradiance in these regions.

Current art mechanisms exist for increasing the dynamic range of an image. Such mechanisms may include capturing multiple images at different exposures at different times (time multiplexing) or using different imaging devices (imager multiplexing) and merging the multiple exposures to generate a higher dynamic range image. The exposure settings of a camera generally include three components: aperture or f-stop (e.g., the size of an aperture permitting light to enter the camera); shutter speed or exposure time (e.g., the length of time that an image sensor is exposed to light); and gain or ISO (e.g., the magnitude of amplification applied to the signal detected by the image sensor). In the case of time multiplexing, different images are captured at different times (e.g., sequentially) and therefore these techniques may can generally be used only with static scenes, as the images may depict motion blur for any objects that move in the scene and/or moving objects may be in different locations in the different images. In addition, capturing multiple exposures using different aperture settings can also cause the appearance of different depths of field, resulting in some parts of the scene showing different levels of focus blur between different exposures. Yet another deficiency may be that capturing multiple exposures with different gain (ISO) settings may cause differences in sensor noise levels between the different exposures.

Aspects of embodiments of the present disclosure relate to systems and methods for capturing a high dynamic range (HDR) image with a single shot (e.g., a single exposure) and systems and methods for adjusting the camera parameters in real time according to the scene properties such that the dynamic range of the camera matches the radiance range of the scene. This is useful in various environments where the radiance range of lighting conditions may be large and may constantly be varying over time. For example, in various manufacturing applications, lighting conditions may change due to shadows cast by objects moving in front of light sources, changes in ambient light due to the movement or usage of machinery or manufacturing (e.g., light from arc welders, reflections from moving metal parts, etc.). Capturing high dynamic range images of a scene enables computer vision systems deployed in these environments to capture more detail, thereby improving the performance of these computer vision systems in performing vision tasks such as object detection, defect detection, object pose detection, object picking, and the like. Embodiments of the present disclosure are also applicable in other imaging contexts, such as capturing HDR images for computer vision tasks in outdoor environments with rapidly changing lighting conditions (e.g., barcode scanning, box scanning, and the like), HDR images for self-driving vehicles encountering high dynamic range scenes (e.g., scenes where part of the street is shaded and part of the street is in direct sunlight), capturing HDR images in hobbyist photography (e.g., imaging a subject in the shade while also capturing the sky on a sunny day or imaging the moon and the stars in a single shot), and the like.

FIG. 1A is a schematic diagram depicting an imaging system 10 including crossed polarizers according to some embodiments of the present disclosure. In the embodiment shown in FIG. 1A, an imaging system (e.g., a camera) includes a processing circuit or controller 12 and a camera matrix or image sensor 14 (e.g., an image sensor such as a complementary metal oxide semiconductor (CMOS) digital image sensor or a charge coupled device (CCD) digital image sensor) having a plurality of light sensitive elements referred to herein as subpixels. The controller 12 may include a processor and memory storing instructions that configure the processor to perform functions to operate the imaging system. For example, the controller 12 may include or be implemented in a central processing unit (CPU), an application processor (AP) of a mobile device such as a smartphone, a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), an image signal processor (ISP), a digital signal processor (DSP), and the like.

Each subpixel of the camera matrix or image sensor 14 is configured to detect light incident thereon and to output a signal (e.g., an analog voltage) representing the amount of light that was detected over the course of an exposure interval. In some embodiments, four adjacent subpixels (e.g., a 2×2 array of subpixels) will be referred to as a pixel 15. FIG. 1A shows an expanded view of a pixel 15 including four subpixels 15 a, 15 b, 15 c, and 15 d. The controller 12 may include or be coupled to read-out circuitry configured to read the signals that were output by the subpixels to capture image data (e.g., digital values) representing the amount of light that was detected by the subpixels during the exposure interval.

In the embodiment shown in FIG. 1A, a micropolarizer mask or polarizer mask 16 is located in front of the image sensor, where the polarizer mask 16 has a repeating pattern of polarizer filters at different angles. In this embodiment, the polarizer filters are aligned with corresponding ones of the subpixels (e.g., there is a 1-to-1 correspondence between the polarizer filters and the subpixels, where each polarizer filter is aligned with one corresponding subpixel). In the embodiment shown in FIG. 1A, the polarizer mask 16 includes a repeating pattern of 2×2 cells or blocks 17 of four polarizer filters 17 a, 17 b, 17 c, and 17 d (e.g., having corresponding polarizing filter angles of 0°, 45°, 90° and 135°, where four subpixels making up a single block or cell 17 are shaded and also shown in expanded view) that are located in front of corresponding 2×2 group of adjacent subpixels 15 a, 15 b, 15 c, and 15 d of a pixel 15 (where one group or cell of subpixels making up a pixel 15 are shaded and also shown in expanded view). Accordingly, first subpixel 15 a receives light filtered by first polarizer filter 17 a (e.g., having a polarizing filter at an angle of 0°), second subpixel 15 b receives light filtered by second polarizer filter 17 b (e.g., having a polarizing filter at an angle of 45°), third subpixel 15 a receives light filtered by third polarizer filter 17 c (e.g., having a polarizing filter at an angle of 90°), and fourth subpixel 15 d receives light filtered by fourth polarizer filter 17 d (e.g., having a polarizing filter at an angle of 135°).

As one example, the Sony® IMX250MZR sensor available from Sony Group Corporation of Tokyo, Japan includes a polarization mask, with a polarization mosaic aligned with the pixel grid of the image sensor 14 in a manner similar to a red-green-blue (RGB) color filter (e.g., a Bayer filter having a red-green-blue-green layout) of a color camera. In a manner similar to how a color filter mosaic filters incoming light based on wavelength such that each pixel in the image sensor 14 receives light in a particular portion of the spectrum (e.g., red, green, or blue) in accordance with the pattern of color filters of the mosaic, a polarization mask 16 using a polarization mosaic of repeating blocks or cells of polarization filters causes light to be filtered based on linear polarization such that different subpixels receive light at different angles of linear polarization (e.g., a block or cell of linear polarizers may have different polarizers oriented at 0°, 45°, 90°, and 135°, or at 0°, 60°, and 120°). Accordingly, a camera of the camera array 10′ may use a polarization mask 16 to concurrently or simultaneously capture light at multiple different linear polarizations (e.g., four different linear polarizations in the case of each cell or block having polarizers oriented at 0°, 45°, 90°, and 135°).

As shown in FIG. 1A, the imaging system 10 further includes a rotatable linear polarizer 18. The rotatable linear polarizer can be rotated to an angle α with respect to a zero direction 19. For the sake of illustration, the zero direction 19 is aligned with the polarizing angle of the first polarizing filters 17 a of the polarization mask 16 (but embodiments of the present disclosure are not limited thereto), and therefore, when the rotatable linear polarizer 18 is set at an angle α of 0°, it is aligned (parallel) with the first polarizing filters 17 a and orthogonal to the third polarizing filters 17 c having linear polarizing filters at 90°.

FIG. 1A further shows that the imaging system 10 further includes imaging optics 20. The imaging optics 20 may include optical components such as a lens system configured to focus light from the scene along an optical axis 21 of the imaging system 10 onto the image sensor 14, structures or an apparatus (e.g., an aperture stop) for controlling the aperture or size of an opening that limits the amount of light passed through the imaging optics 20. While FIG. 1A shows the imaging optics 20 as being located between the rotatable linear polarizer 18 and the image sensor 14, embodiments of the present disclosure are not limited thereto. For example, in some embodiments, such as the embodiment shown in FIG. 1B, the rotatable linear polarizer 18 is located between the imaging optics 20 and the image sensor 14. The imaging system shown in FIG. 1B may otherwise operate substantially similarly to that shown in FIG. 1A.

In various embodiments of the present disclosure, various types of rotatable linear polarizers 18 may be used in the imaging system 10 to implement high dynamic range imaging using crossed polarization. Some examples of types of rotatable linear polarizers 18 include electronically-controlled mechanically rotatable linear polarizers and liquid crystal polarization rotators.

FIG. 1C is a schematic diagram depicting a rotatable linear polarizer implemented as an electronically controlled mechanically rotatable linear polarizer according to some embodiments of the present disclosure. For example, in some embodiments, an electronically controlled mechanically rotatable linear polarizer includes an electrical motor 22 configured to mechanically rotate a linear polarizer 24 (e.g., around the optical axis 21) to adjust or change the angle α of linear polarization of the linear polarizer 24 relative to a zero direction 19 of the polarization system of the imaging system 10. For example, the electric motor 22 may include permanent magnet DC motors connected to a gear train meshed with teeth on an edge of the edge of the linear polarizer 24. As another example, the electric motor 22 may include a piezoelectric ring motor (e.g., a piezoelectric revolving resonator and ultrasonic motor, see, e.g., U.S. Pat. Nos. 4,868,446 and 5,008,581). As such, an electronically controlled mechanically rotatable linear polarizer may be used to implement a rotatable linear polarizer 18 configured to control the angle of linear polarization of light passing through the imaging system 10 according to some embodiments of the present disclosure.

FIG. 1D is a schematic diagram depicting a rotatable linear polarizer implemented as a liquid crystal polarization rotator according to some embodiments of the present disclosure. Some examples of liquid crystal polarization rotators include the LCR1 line of liquid crystal polarization rotators available from Thorlabs of Newton, N.J., United States. For example, a rotatable linear polarizer 18 in the embodiment shown in FIG. 1D includes a fixed linear polarizer 24 that transmits polarized light 25 aligned with its polarization angle. The polarization angle of the fixed linear polarizer 24 is set such that it is 45° from the fast and slow axes of a liquid crystal retarder 26. The difference in retardance between the fast and slow axes of the liquid crystal retarder 26 converts the linear input polarization of the polarized light 25 into an elliptical polarization that has its major and minor axes oriented at 45° with respect to the fast and slow axes of the liquid crystal retarder. A liquid crystal controller 27 is configured to control the fast and slow axes of the liquid crystal retarder 26, thereby enabling controlling the orientation of the major and minor axes of the elliptical polarization of the light that exits the liquid crystal retarder 26. The elliptically polarized light then passes through a quarter-wave plate 28, which converts the elliptically polarized light into linearly polarized light 29 at an angle α with respect to the zero direction 19 of the polarization system of the imaging system 10. As such, a liquid crystal polarization rotator may be used to implement a rotatable linear polarizer 18 configured to control the angle of linear polarization of light passing through the imaging system 10 according to some embodiments of the present disclosure.

FIGS. 1A and 1B show example embodiments of imaging systems 10 that include a polarizer system including crossed polarizers, where each subpixel (e.g., 15 a, 15 b, 15 c, and 15 d) of a given pixel 15 is behind a polarizer system that includes the rotatable linear polarizer 18 and the corresponding (fixed) polarizing filter (e.g., 17 a, 17 b, 17 c, and 17 d) of the polarization mask 16. Generally, the radiance of light passing through a pair of crossed polarizers is a function of the relative angles of the two polarizers, where the transmission through the polarizer system is at a maximum when the pair of crossed polarizers is aligned (e.g., at the same angle) and at a minimum when the crossed polarizers have polarizing angles that are orthogonal to one another (e.g., 90° apart). More specifically if the difference between the two angles is given by θ, and the light incident on the polarizer system at one block 17 has radiance I₀, then, in the case of unpolarized light, the radiance of light transmitted through the crossed polarizers is proportional to cos² θ, that is:

$\frac{1}{2}I_{0}\cos^{2}\theta$

In the case described above where each pixel of four subpixels is associated four corresponding polarizing filters 17 a, 17 b, 17 c, and 17 d at respective angles of 0°, 45°, 90°, and 135° and assuming that the radiance is the same value I₀ on all four polarizing filters, then the radiances of light transmitted through the polarizer system (the rotatable polarizer at angle α and the polarizing filters of the polarization mask) can be given by:

$I_{1} = {\frac{1}{2}I_{0}{\cos^{2}\left( {{0{^\circ}} - \alpha} \right)}}$ $I_{2} = {\frac{1}{2}I_{0}{\cos^{2}\left( {{45{^\circ}} - \alpha} \right)}}$ $I_{3} = {\frac{1}{2}I_{0}{\cos^{2}\left( {{90{^\circ}} - \alpha} \right)}}$ $I_{4} = {\frac{1}{2}I_{0}{\cos^{2}\left( {{135{^\circ}} - \alpha} \right)}}$

where I₁ is the radiance through the first polarizing filter 17 a at angle 0°, I₂ is the radiance through the second polarizing filter 17 b at angle 45°, I₃ is the radiance through the third polarizing filter 17 c at angle 90°, and I₄ is the radiance through the fourth polarizing filter 17 d at angle 135°.

A similar calculation would apply in the case of polarized or partly polarized light, with the exception that the coefficient would be a value different from ½, depending on the angle of linear polarization (AOLP) and the degree of linear polarization (DOLP) of the light.

The above equations assume complete extinction in the case of perpendicular crossed polarizers (e.g., no transmission when the polarizers are 90° apart). In reality, the extinction ratio may vary based on the type of polarizer, and may have values such as 100:1 to 10,000:1, and assume that the influence of the imaging optics 20 on radiance is negligible (e.g., assumes that the impact of the imaging optics 20 is the same for light of any polarization).

If the angle between the rotatable linear polarizer 18 and the polarizing filters of the polarizer mask 16 at certain subpixels is close to 90°, then the range between the darkest and brightest possible radiance values that could be captured with a single shot can reach 10⁹ and more. This is due to the fact that no polarizer is perfect and there is a certain extinction rate. Therefore, even for crossed polarizers some amount of light may pass. If the angle between the rotatable linear polarizer 18 and the polarizing filters of the polarizer mask 16 is in-between 0° and 45° degrees (e.g., at 22.5°), the resulting range of the system may be significantly lower, e.g., about 10¹.

FIG. 2 is a graph depicting the dynamic range of an imaging system according to one embodiment of the present disclosure as a function of the angle of the rotatable polarizer. FIG. 2 was produced by dividing the maximum radiance among I₁, I₂, I₃, I₄ above by the minimum radiance among I₁, I₂, I₃, I₄ for angles α from 0 to 22.5°, assuming unpolarized light and assuming an imperfect extinction rate (e.g., incomplete extinction when the crossed polarizers are orthogonal). As seen in FIG. 2 , the dynamic range is at a maximum (e.g., the y-axis of the graph is labeled up to 10⁴, but continues to increase for smaller angles α, as constrained by the extinction ratio in a state where the rotatable linear polarizer 18 is crossed at 90° with portions of the polarization mask 16, such as the extinction ratio between the linear polarizers of the polarization mask oriented at 90° and the rotatable linear polarizer 18 when the rotatable linear polarizer is at an angle α of 0°) when the rotatable linear polarizer 18 is aligned with one of the polarizing filters 17 (e.g., at 0°, the rotatable linear polarizer 18 is aligned with first polarizing filter 17 a) and perpendicular to another polarizing filter (e.g., at 0°, the rotatable linear polarizer is perpendicular to the third polarizing filter 17 c which is at 90°). The dynamic range is at a minimum (e.g., 10¹) when the rotatable linear polarizer 18 is at 22.5°.

While FIG. 2 merely shows changing the angle α of the rotatable linear polarizer 18 over a range of 0° to 22.5°, embodiments of the present disclosure are not limited thereto. As noted above, in circumstances where the light detected by the imaging system is polarized or partially polarized, setting the angle α to angles in the range of 22.5° to 180° may result in different levels of attenuation and different changes in the dynamic range, in accordance with the polarization characteristics (e.g., AOLP and DOLP) of the incident light. In addition, the above statements assume that the angles of linear polarization of the polarizer mask 16 are at 0°, 45°, 90°, and 135°. In other embodiments with polarization masks with different angles, the angle α of the rotatable linear polarizer 18 may be changed over a different range of angles to control the dynamic range of the imaging system. For example, in cases where the polarizer mask 16 has a repeating pattern of linear polarizers at 0°, 60°, and 120°, the angle α of the rotatable linear polarizer 18 may be varied over a range of 0° to 30°. In other words, where the polarization mask 16 includes a repeating mosaic of linear polarizers spaced at angles of ϕ degrees, the angle α of the rotatable linear polarizer 18 may be varied over a range of ϕ/2.

The measurement made at each subpixel of the camera matrix or image sensor 14 can therefore be represented as:

L _(i,C) =f(I _(i) *t ₀)=f(I ₀ cos²(Θ_(i)−α)t ₀)

where i is an index referring to an i-th subpixel behind an i-th polarizing filter at an angle of Θ_(i), α is the angle of the rotatable linear polarizer 18, I_(i) is the radiance of light passing on the i-th subpixel, I₀ is the radiance of light incident on the polarization system at the i-th subpixel, t₀ is the exposure time or length of the exposure (and assumes that the radiance is constant over the exposure time or exposure interval), and f is the camera response function. The camera response function f maps the amount of light incident on the subpixel over the exposure time (I_(i)*t₀) to a scalar value, such as an 8-bit digital value from 0 to 255 (embodiments of the present disclosure are not limited thereto and may represent values using fewer than 8-bits or more than 8-bits).

Values output by different subpixels that are located behind linear polarizers of the polarizer mask having the same polarization angle may be grouped into a same polarization channel, such that each polarization channel is an image of detected radiances passing through a same configuration of the polarization system. For example, all subpixels that are located behind linear polarizers at 0° may be grouped together into a 0° channel, and all subpixels that are located behind linear polarizers at 45° may be grouped together into a 45° channel. This is similar to the red, green, and blue channels of a color camera, where the red channel is an image representing the amounts of light detected by the subpixels that are behind red color filters, the blue channel is an image representing the amounts of light detected by the subpixels that are behind blue color filters, and the green channel is an image representing the amounts of light detected by the subpixels that are behind green color filters.

As such, changing the angle α of the rotatable linear polarizer 18 causes non-linear changes in the attenuation of the radiance of the light by the polarization system (or, equivalently, the transmission of light through the polarization system) through the differently oriented linear polarizers of a block 17 of fixed linear polarizers of the polarizer mask 16, at least because the transmission through the sets of cross polarizers is non-linear with respect to the angle between the polarizers (e.g., is proportional to cost² θ, as noted above and depends on the angle of linear polarization and degree of linear polarization of the light). More concretely, as the rotatable linear polarizer 18 changes its angle α to be closer in angle to the linear polarizer of a particular subpixel, the transmission of light to that subpixel increases. For example, as the angle α of the rotatable linear polarizer 18 decreases from 22.5° to 0°, the transmission through the fixed linear polarizer filters 17 a having a filter angle at 0° increases, thereby causing the range of the imaging system to expand downward to regions of lower radiance (e.g., to detect light in darker portions of the scene without underexposure or falling below the noise floor of the subpixel). Likewise, as the rotatable linear polarizer 18 changes its angle α to be closer in angle to being perpendicular to the linear polarizer of a particular subpixel, then the transmission of light to that subpixel decreases. For example, as the angle α of the rotatable linear polarizer 18 decreases from 22.5° to 0°, the transmission through the fixed linear polarizer filters 17 c having a filter angle at 90° decreases, thereby causing the range of the imaging system to expand upward to regions of higher radiance (e.g., to detect light in brighter portions of the scene without overexposure or saturation of the subpixel).

Likewise, adjusting the angle α of the rotatable linear polarizer 18 toward more median angles (e.g., increasing the angle α from 0° toward 22.5°) decreases or contracts the dynamic range of the imaging system, such as by reducing the differences in the degree of attenuation of light by the polarization system at the different subpixels (e.g., the differences in the degrees of attenuation represented in the different polarization channels).

Accordingly, embodiments of the present disclosure enable the dynamic control of the dynamic range of an imaging system by setting the angle α of linear polarization of a rotatable linear polarizer 18 placed in an optical path of an imaging system with a polarization mask 16 having a mosaic of fixed polarizers at different angles of polarization. In particular, the cos² θ attenuation factor, which is controllable through controlling the angle α of the rotatable linear polarizer 18, provides an additional method of controlling the amount of light that reaches the image sensor 14, independently of exposure settings such as exposure time (or the length of the exposure interval), aperture, and sensor gain.

FIG. 3 is a flowchart depicting a method 300 for controlling an imaging system to increase the dynamic range of the imaging system based on the range of radiance of a scene being imaged by the imaging system according to one embodiment of the present disclosure. In the embodiment shown in FIG. 3 , it is assumed that the current dynamic range of the imaging system is smaller than the dynamic range of the radiance I of the scene being imaged. In some embodiments of the present disclosure, the method 300 is implemented by a controller 12 that includes a processing circuit (e.g., a processor and memory) configured to control the operation of the imaging system.

FIG. 4A is a graph comparing the radiance range of a scene to an initial dynamic range of the imaging system before beginning adjustment. In more detail, FIG. 4A shows three adjacent pixels (labeled 0, 1, and 2) along one row of an image extending along the x-direction, along with the radiance I of the light from the scene and directed toward those three pixels. (As noted above, each pixel includes a plurality of subpixels where each subpixel is located behind a polarizing filter with a different angle of linear polarization than the other subpixels.) For the sake of illustration, the radiance I at each pixel x will be denoted as I(x), and in FIG. 4A, the scene has minimum radiance at I(1) and maximum radiance at I(0) and I(2). Accordingly, the dynamic range of the radiance I of the scene is indicated by scene range 410, as shown by double headed arrows extending between the minimum radiance and the maximum radiance. As noted above, the initial dynamic range 421 of the imaging system (as controlled by the angle of the rotatable linear polarizer 18) before beginning adjustment is smaller than the dynamic range 410 of the radiance of the scene.

Referring to FIG. 3 , in operation 330, the controller 12 determines whether the current image includes underexposed pixels and overexposed pixels. In the context of this discussion, a pixel is underexposed when all of its subpixels output values that are at or near a minimum of its range (e.g., at or near 0) or, in other words, the values at that pixel are at or near the minimum across all of the polarization channels. Likewise, a pixel is overexposed when all of its subpixels are at or near a maximum of its output range (e.g., at or near 255) or, in other words, the values at that pixel are at or near the maximum across all of the polarization channels. A given pixel is not overexposed or underexposed when at least one of its subpixels (or one of its channels) is neither saturated nor below its noise floor. When at least some pixels in the image are underexposed and at least some pixels are overexposed, then the current dynamic range 421 of the imaging system 10 is smaller than the dynamic range of the radiance I of the scene being imaged. In some embodiments, the controller 12 determines whether there are some overexposed and some underexposed pixels when at least some of the pixels are outside of set thresholds (e.g., a lower threshold at or near a value of 0 and an upper threshold at or near a value of 255, in the case of 8-bit values).

As such, in this circumstance, in operation 350 the controller 12 increases the range of the imaging system by rotating the rotatable linear polarizer 18 (e.g., decreasing the angle α from 22.5° toward 0°).

The controller then returns to operation 310 to capture another image with the imaging system 10 configured to an adjusted range, as set by the new angle α of the rotatable linear polarizer 18. Accordingly, some aspects of embodiments of the present disclosure relate to iteratively adjusting the angle of the rotatable linear polarizer 18 and therefore adjusting the dynamic range of the imaging system 10 based on the dynamic range of the scene being imaged until the dynamic range of the imaging system is equal to or larger than the dynamic range of the radiance of the scene (e.g., when an image captured based on the adjusted dynamic range does not include any underexposed pixels and does not include any overexposed pixels).

In some embodiments, in operation 350, the controller 12 determines the size of the adjustment Δα to the polarizer angle α in accordance with a function, such as by dividing the current angle α by two toward an angle of closer alignment with one of the fixed polarizer filters (e.g., the polarizer filters 17 a at angle 0°) of the polarizer mask 16. For example, when starting at a smallest dynamic range with the angle s at 22.5°, increasing the dynamic range by dividing the angle α by 2 to a new angle of 12.25°, dividing the angle again by two at a later iteration to 6.125°, and the like. As another example, in some embodiments the controller 12 controls the polarizer angle based on a function based on the number of underexposed and overexposed pixels in the current image (e.g., Δα may be larger when there is a larger number of overexposed pixels and underexposed pixels) or may be based on fixed angle changes (e.g., a step change of 2.25°). In some circumstances, the dynamic range of the scene may be larger than the maximum possible dynamic range of the imaging system (e.g., as constrained by the extinction ratio of the polarization system), in which case an image may be captured with some underexposed pixels and some overexposed pixels at the maximum dynamic range setting (e.g., with the rotatable linear polarizer 18 at an angle α of 0°).

FIG. 4B is a graph showing range of a scene to a dynamic range of the imaging system after adjusting the rotatable polarizer according to one embodiment of the present disclosure. As shown in FIG. 4B, adjusting the angle α of the rotatable linear polarizer 18 expands the dynamic range 423 of the imaging system 10 such that it is sufficiently large to encompass the full dynamic range of the radiance I 410 of the scene being imaged. In particular, increasing the dynamic range 423 of the imaging system 10 increases the attenuation of light at subpixels that have larger differences in angle between their corresponding fixed polarizer filters 17 and the angle α of the rotatable linear polarizer 18, thereby enabling measurements without saturation of the sensor at pixels (or image locations) with high scene radiance I and also decreases the attenuation of light at subpixels that have smaller differences in angle between their corresponding fixed polarizer filters 17 and the angle α of the rotatable linear polarizer 18, thereby enabling measurements above the noise floor of the sensor at pixels (or image locations) with low scene radiance I.

In some circumstances, expanding the dynamic range of the imaging system 10 until there are no underexposed pixels and no overexposed pixels may result in poor exposure settings. Generally, the controller 12 of an imaging system may automatically adjust its exposure settings (e.g., aperture, shutter speed or exposure time, and sensor gain or ISO) to reduce the number of overexposed and underexposed pixels. For example, when the majority of the pixels of the image are underexposed, an automatic exposure algorithm may gradually increase the exposure settings (e.g., increase the aperture, increase the exposure time, and/or increase the sensor gain) so that most of the pixels are within the dynamic range of the camera. Likewise, when the majority of the pixels are overexposed, the automatic exposure algorithm may gradually decrease the exposure settings. In some circumstances, the automatic exposure algorithm may give different weight to different parts of the image when performing the adjustment, such as weighting pixels based on distance from the center of the image, therefore allowing some overexposed or underexposed pixels at the edges of the image in exchange for better exposure near the center of the image.

Accordingly, in operation 370, the controller 12 may adjust the exposure settings of the imaging system (e.g., controlling the imaging optics 20 and/or the gain when reading data from the image sensor 14) based on the current image (as well as, in some embodiments, additional images captured during the exposure setting adjustment operation). (Note that changing the exposure settings such as the aperture, shutter speed, and gain of an imaging system generally causes the range to shift upward or downward to capture different parts of the total scene radiance, but generally does not cause a change in the size of the dynamic range).

In operation 380, the controller 12 determines whether the current dynamic range of the imaging system matches the dynamic range of the radiance I of the scene. In the example shown in FIG. 4B, after adjustment, the dynamic range 423 of the imaging system 10 is larger than the dynamic range 410 of the radiance k of the scene. (This overcompensation may have occurred due to the step size of the changes Δα in the angle a of the rotatable linear polarizer 18 and/or due to ambiguities in the degree to which pixels were overexposed or underexposed, e.g., how much the radiance was above the saturation point and how much the radiance was below the floor) While such an arrangement would be capable of imaging such a scene, some details may be lost during capture due to the mismatch in the range. In particular, fine differences in the radiances of light at two different pixels may fail to be captured by the imaging system because both radiances may be mapped to the same discrete value by the camera response function f as part of the process of converting the analog signals to digital values (e.g., quantization error).

As such, in some embodiments of the present disclosure, the controller 12 detects a mismatch in the current dynamic range of the imaging system 10 and the dynamic range of the radiance I of the scene when the pixels of highest radiance are lower than the maximum detectable radiance of the sensor (e.g., have values below 255 by more than a threshold amount) and/or when the pixels of lowest radiance are higher than a lowest detectable radiance of the sensor (e.g., have values higher than 0 by more than a threshold amount). In such a case, in operation 390, the controller 12 decreases the dynamic range of the imaging system 10 by adjusting the angle of the rotatable linear polarizer 18.

FIG. 4C is a graph comparing the radiance range of a scene to a dynamic range of the imaging system after adjusting the rotatable polarizer and the exposure settings of the imaging system according to one embodiment of the present disclosure. As shown in FIG. 4C, after adjusting the angle of the rotatable linear polarizer 18 to decrease the dynamic range of the imaging system 10, the updated dynamic range 425 matches the dynamic range 410 of the radiance of the scene, thereby improving the quality of the images captured by the imaging system 10.

In the arrangement shown above in FIG. 4A, the initial dynamic range 421 off the imaging system 10 was illustrated as being smaller than the dynamic range of the radiance I of the scene and between the minimum and maximum radiances of the scene. However, embodiments of the present disclosure are not limited to these particular initial conditions.

For example, in a circumstance where the initial dynamic range of the imaging system 10 is smaller than the dynamic range of the radiance I 410 of the scene and the exposure settings cause the initial range of the imaging system to be at or below the lower end or at or above the above upper end of the dynamic range of the radiance I of the scene, then the initial captured image may include only underexposed pixels or only overexposed pixels. In such a circumstance, in some embodiments, the controller 12 adjusts the exposure settings of the imaging system 10 such that the image includes both overexposed pixels and underexposed pixels prior to beginning adjustment of the range of the imaging system in accordance with the method shown in FIG. 3 .

Similarly, in some circumstances, the initial dynamic range of the imaging system 10 may be larger than the dynamic range of the radiance I of the scene. This may occur when the imaging system (e.g., a video camera) transitions from imaging a scene with a high dynamic range (e.g., outdoors with deeply shaded regions) to a scene with lower dynamic range (e.g., indoors with artificial lighting). In these circumstances, the controller 12 may automatically adjust the polarizer angle to decrease range in accordance with operation 390 in response to detecting a range mismatch in the captured images and the current scene, in accordance with operation 380.

As such, after adjusting both the dynamic range of the imaging system 10 and the exposure settings of the imaging system 10 to the current dynamic range of the scene, the resulting captured image having a matched range is then output results in capturing an image with multiple polarization channels, where each channel is subject to a different level of attenuation based on the angle α of the rotatable linear polarizer 18. The multiple polarization channels can be considered as corresponding to different exposures in a comparative high dynamic range imaging technique. Accordingly, in some embodiments, such as for viewing of the captured high dynamic range image on a standard display device, techniques for combining different exposures (e.g., tone mapping) can be used to combine the different polarization channels to generate an output image having the appearance of a high dynamic range image. In some embodiments, the high dynamic range image is re-mapped into a data format that is capable of representing the full dynamic range (e.g., images having higher bit depth than the bit depth of the captured high dynamic range image). In some embodiments, the high dynamic range image is supplied as input for further processing, such as in the form of input to a computer vision or robotic vision system for controlling a robotic system to interact with (e.g., pick up, place, and/or manipulate) objects in the imaged scene.

Various aspects of embodiments of the present disclosure are described above with respect to a monochrome imaging system, e.g., an imaging system that does not include a color filter such as a Bayer filter in the optical path. However, embodiments of the present disclosure are not limited thereto. For example, in some embodiments, the optical path further includes a mosaic color filter such that different subpixels may detect light in different portions of the visible spectrum (e.g., red, green, or blue light) and/or portions of the invisible spectrum (e.g., infrared light).

FIG. 5 is a schematic diagram depicting an imaging system including crossed polarizers and a color filter according to some embodiments of the present disclosure. The embodiment shown in FIG. 5 is substantially similar to the embodiment shown in FIG. 1A, and therefore description of similar parts will not be repeated herein. While an additional color filter is shown as an addition to the arrangement of FIG. 1A, embodiments of the present disclosure are not limited thereto and a color filter may be used in combinations with other systems such as where the positions of the rotatable linear polarizer 18 and the imaging optics 20 along the optical axis 21 are switched. In some embodiments of the present disclosure, a color filter 500 is arranged in the optical path or optical axis 21 such that each pixel 15 of four subpixels 15 a, 15 b, 15 c, and 15 d (behind differently oriented linear polarizers 17 a, 17 b, 17 c, 17 d of a cell 17 of linear polarizers of the polarizer mask 16) are arranged behind a single color filter 502 or separate color filters configured to transmit light of the same color (e.g., green light). In the arrangement shown in FIG. 5 , the color filter 500 has a mosaic of color filters arranged in a Bayer pattern, where a vertical shading pattern indicates green color filters, a horizontal shading pattern indicates red color filters, and a dot shading pattern indicate blue color filters). As such, in this embodiment, the subpixels of a given pixel 15 receive light of the same color, and adjacent pixels 15 (groups of 4 subpixels) detect light of different colors.

In some embodiments of the present disclosure, the color filter 500 is configured to have a repeating pattern or mosaic pattern where each color filter corresponds to a different subpixel (e.g., 15 a, 15 b, 15 c, 15 d) of the pixels and the polarization mask 16 is configured to have a repeating pattern of polarizer filters where each polarizer filter corresponds to one pixel 15 (e.g., where all four subpixels 15 a, 15 b, 15 c, 15 d of a pixel are behind a same linear polarizer of the polarization mask and behind different color filters).

As such, aspects of embodiments of the present disclosure relate to systems and methods for performing high dynamic range imaging of a scene using crossed polarizers. In some embodiments, a polarization mask includes a repeating mosaic of linear polarizers at different orientations (e.g., 0°, 45°, 90°, and 135°) aligned with different subpixels of an image sensor. A rotatable linear polarizer in the optical path of the imaging system is used to control controlling the angle of polarization of light reaching the polarization mask, thereby causing different relative angles θ between the light incident on the polarization mask and the linear polarizers of the polarization mask, thereby resulting in different degrees of attenuation of the light passing through the mask approximately proportional to a cos² θ factor. Accordingly, controlling the angle of the rotatable linear polarizer according to some embodiments of the present disclosure controls the overall dynamic range of the imaging system, where greater differences in the degree of attenuation of light incident on different subpixels results in a larger dynamic range of the imaging system and smaller differences in the degree of attenuation of light incident on different subpixels results in a smaller dynamic range of the imaging system.

Referring back to FIG. 1D, in some embodiments of the present disclosure, the liquid crystal polarization rotator includes a matrix of liquid crystal retarders, where individual liquid crystal retarders in the matrix are individually controllable to rotate light within a range of angles (in a manner similar to the pixels of a liquid crystal display). Accordingly, these embodiments of the present disclosure enable local control of the dynamic range of the imaging system in different parts of the captured image.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof. 

What is claimed is:
 1. An imaging system comprising: an image sensor comprising a plurality of subpixels grouped into a plurality of pixels; a polarization system comprising: a rotatable linear polarizer; and a polarizer mask comprising a plurality of polarizer filters, the polarizer filters being aligned with corresponding ones of the subpixels, the subpixels of a pixel of the plurality of pixels being located behind polarizer filters at different angles of linear polarization; and imaging optics configured to focus light from a scene onto the image sensor.
 2. The imaging system of claim 1, further comprising a processing circuit configured to: control the image sensor to capture an image of the scene; determine that the image includes underexposed pixels and overexposed pixels; and adjust a dynamic range of the imaging system by controlling an angle of the rotatable linear polarizer.
 3. The imaging system of claim 2, wherein the processing circuit is further configured to iteratively adjust the dynamic range of the imaging system until the dynamic range of the imaging system is equal to or larger than a dynamic range of the scene.
 4. The imaging system of claim 2, wherein the processing circuit is further configured to control a plurality of exposure settings in accordance with the dynamic range of the imaging system and the dynamic range of the scene.
 5. The imaging system of claim 2, wherein the processing circuit is further configured to compute a high dynamic range image from a plurality of polarization channels of captured image.
 6. The imaging system of claim 1, wherein the rotatable linear polarizer comprises an electronically controlled mechanically rotatable linear polarizer.
 7. The imaging system of claim 1, wherein the rotatable linear polarizer comprises a liquid crystal polarization rotator.
 8. The imaging system of claim 1, wherein the rotatable linear polarizer is in front of the imaging optics.
 9. The imaging system of claim 1, wherein the rotatable linear polarizer is behind the imaging optics.
 10. The imaging system of claim 1, further comprising a color filter.
 11. The imaging system of claim 1, wherein the polarizer mask comprises a repeating mosaic of the polarizer filters at the different angles of linear polarization, the different angles of linear polarization comprising a first angle 0°, a second angle of 45°, a third angle of 90°, and a fourth angle 135°, and wherein a pixel of the pixels comprises: a first subpixel behind a first polarizer filter of the polarizer mask at the first angle of 0°; a second subpixel behind a second polarizer filter of the polarizer mask at the second angle of 45°; a third subpixel behind a third polarizer filter of the polarizer mask at the third angle of 90°; and a fourth subpixel behind a fourth polarizer filter of the polarizer mask at the fourth angle of 135°.
 12. The imaging system of claim 11, further comprising a processing circuit configured to control the rotatable linear polarizer to rotate in a range from 0° to 22.5°.
 13. A method for capturing images comprising: controlling, by a processing circuit, an image sensor of an imaging system to capture an image, the image sensor comprising a plurality of subpixels grouped into a plurality of pixels, the imaging system further comprising: a polarization system comprising: a rotatable linear polarizer; and a polarizer mask comprising a plurality of polarizer filters, the polarizer filters being aligned with corresponding ones of the subpixels, the subpixels of a pixel of the plurality of pixels being located behind polarizer filters at different angles of linear polarization; and imaging optics configured to focus light from a scene onto the image sensor; determining, by the processing circuit, that the image includes underexposed pixels and overexposed pixels; and adjusting, by the processing circuit, a dynamic range of the imaging system by controlling an angle of the rotatable linear polarizer.
 14. The method of claim 13, further comprising iteratively adjusting the dynamic range of the imaging system until the dynamic range of the imaging system is equal to or larger than a dynamic range of the scene.
 15. The method of claim 13, further comprising controlling a plurality of exposure settings in accordance with the dynamic range of the imaging system and the dynamic range of the scene.
 16. The method of claim 13, further comprising computing a high dynamic range image from a plurality of polarization channels of captured image.
 17. The method claim 13, wherein the rotatable linear polarizer comprises an electronically controlled mechanically rotatable linear polarizer.
 18. The method of claim 13, wherein the rotatable linear polarizer comprises a liquid crystal polarization rotator.
 19. The method of claim 13, wherein the rotatable linear polarizer is in front of the imaging optics.
 20. The method of claim 13, wherein the rotatable linear polarizer is behind the imaging optics.
 21. The method of claim 13, wherein the imaging system further comprises a color filter.
 22. The method of claim 13, wherein the polarizer mask comprises a repeating mosaic of the polarizer filters at the different angles of linear polarization, the different angles of linear polarization comprising a first angle 0°, a second angle of 45°, a third angle of 90°, and a fourth angle 135°, and wherein a pixel of the pixels comprises: a first subpixel behind a first polarizer filter of the polarizer mask at the first angle of 0°; a second subpixel behind a second polarizer filter of the polarizer mask at the second angle of 45°; a third subpixel behind a third polarizer filter of the polarizer mask at the third angle of 90°; and a fourth subpixel behind a fourth polarizer filter of the polarizer mask at the fourth angle of 135°.
 23. The method of claim 22, further comprising controlling the rotatable linear polarizer to rotate in a range from 0° to 22.5°. 